Click the Flag. Scroll through the list.
It was very easy to make this project (it took 2 minutes). The rules for a username come from a simple Google search, and they are: 1) You can only use letters (uppercase/lowercase), numbers (0-9), and underscores(the symbol next to the 0, _). 2) It can be as small as 3 characters or as long as 16 characters. 3) It must be different from all other people's Minecraft usernames (changing a letter from lowercase to uppercase or uppercase to lowercase doesn't count as a new name). Thanks Minecraft for allowing username changes!